This invention relates to an improved metal treatment process, and more particularly, to a new and improved method of treating phosphated metal surfaces to provide more durable and rust-inhibiting coatings. The invention relates particularly to the post treatment of phosphated metal surfaces with an aqueous solution of a water-soluble organic titanium chelate which forms a water-insoluble deposit on the phosphated metal surface.
It is well known in the metal finishing art that metal surfaces such as aluminum, ferrous, galvanized ferrous and zinc surfaces may be coated with an inorganic phosphate by contacting them with an aqueous phosphating solution. The phosphate coating protects the metal surface to a limited extent against corrosion and serves primarily as an excellent base for the later application of corrosion-inhibiting compositions and siccative organic coating compositions such as paint, lacquer, varnish, primers, synthetic resins, enamel, and the like.
The inorganic phosphate coatings generally are formed on a metal surface by means of aqueous solutions which contain phosphate ion and, optionally, certain auxiliary ions including metallic ions such as sodium, manganese, zinc, cadmium, copper, lead, calcium-zinc and antimony ions. These aqueous solutions also may contain non-metallic ions such as ammonium, chloride, bromide, fluoride, nitrate, sulfate, and borate ions. These auxiliary ions influence the reaction with the metal surface, modify the character of the phosphate coating, and adapt it for a wide variety of applications. Other auxiliary agents such as oxidizing agents, coloring agents, and metal cleaning agents also may be incorporated in the phosphating solution.
As mentioned above, inorganic phosphate coatings provide an excellent base for the application of siccative organic coatings such as paints or lacquers. The provision of such phosphate coatings has been found to improve both the adhesion of the paint or lacquer film to the metal surface and the corrosion resistance of the painted metal.
Solvent-base siccative organic coating compositions have been applied to metal surfaces such as by spraying, dipping, rolling centrifuged dip-spinning, etc. Water-soluble resin base paints and lacquers can be applied by electrophoresis. The electrophoretic application of paint and lacquer involves the phenomena of electro-osmosis and electrolysis, as well as electrophoresis. In this method, an electric current is passed through the paint or lacquer solution while the article to be painted is made an electrode, usually the anode, in the paint or lacquer.
Although the adhesion of the siccative organic coating to the metal surface is improved by the phosphate coating, it has been noted, for example, where ferrous metal, galvanized ferrous metal or phosphated ferrous metal parts are provided with a siccative top coat of lacquer or enamel and such top coat is scratched or scored during, for example, handling, forming or assembling operations, the metal substrate becomes a focal point for corrosion and for a phenomenon known as "undercutting". Undercutting, or the loosening of the top-coat in areas adjacent to a scratch or score causes a progressive flaking of the top-coat from the affected area. In severe cases, the undercutting may extend an inch or more from each side of the scratch or score, causing a loosening and subsequent flaking of the top-coat from a substantial portion, if not all, of the metal article. The undercutting also results in a reduction of the desirable corrosion-resistance properties.
It has been suggested in the prior art that the problem of undercutting can be minimized, and the corrosion-proofing properties of siccative coated metal surfaces improved by treating the phosphated metal surface with various chromium-containing acidic solutions prior to the application of the siccative coating. Aqueous solutions containing hexavalent or trivalent chromium compounds, or mixtures of hexavalent and trivalent chromium compounds have been suggested as useful chromium treatments. The chromic acid rinse solutions appear to "seal" the phosphate coating and improve its utility as a base for the application of siccative organic coatings. However, the use of chromium solutions does result in environmental and health problems created by the toxic chromium compounds. Hexavalent chromium compounds are known to be lethal, and the discharge of trivalent chromium compounds as green waste materials is objectionable.
Chromium free treatment of phosphate coatings has been suggested in U.S. Pat. Nos. 4,110,129; 4,182,637; 4,264,378 and 4,362,577. U.S. Pat. No. 4,110,129 describes the use of an aqueous solution containing a water soluble titanium compound such as titanium fluoride, titanium sodium fluoride or potassium titanyl oxalate and at least one adjuvant compound such as phosphoric acid, phytic acid, or tannin and hydrogen peroxide. U.S. Pat. No. 4,182,637 describes a rinse containing the combination of citric acid and sodium nitrite to enhance corrosion protection. U.S. Pat. No. 4,362,577 describes an aqueous acidic rinse containing hypophosphorous acid, salts of hypophosphorous acid or sodium hypophosphate. U.S. Pat. No. 4,264,378 describes a rinse containing phosphate, a metal cation, and molybdate, vanadate, niobate or tantalate ions. The inventions described in these four patents have a common shortcoming. They do not form insoluble complexes with phosphate and are therefore not suitable for use with electrodeposited paint. It is essential not to carry soluble salts into the paint bath because they throw the electrodeposition process out of balance. Therefore a rinse with deionized water is essential before painting. It follows that the sealing rinse must form an insoluble complex with phosphate, and not be removed by a water rinse, if it is to be compatible with electrodeposited paint.
Organic titanates have often been used to bind together two different materials. U.S. Pat. No. 2,751,314 describes the use of organic titanates to bind silicone polymers to solid surfaces. U.S. Pat. No. 2,838,418 describes how the adhesion of paint to glass is improved if the glass is treated with tetralkyl titanate before painting. U.S. Pat. No. 2,948,641 describes how the adhesion of printing ink to polyethylene is increased when an organic titanate is incorporated into the polymer. Many similar applications may be found in the literature of manufacturers of organic titanates.